As to hiit vs liss (low intensity steady state), you will burn more calories doing hiit although majority of those calories will come from carbs, or if no carbs are readily available then you body will burn muscle for fuel. Liss on the other hand uses stored fat for the majority of fuel.

You could be less than 10% bodyfat but not have much muscle and it wouldn't look anywhere near as aesthetic as someone with muscle at 12-13%.